|Publication number||US7088051 B1|
|Application number||US 11/102,354|
|Publication date||Aug 8, 2006|
|Filing date||Apr 8, 2005|
|Priority date||Apr 8, 2005|
|Publication number||102354, 11102354, US 7088051 B1, US 7088051B1, US-B1-7088051, US7088051 B1, US7088051B1|
|Inventors||Ronald S. Cok|
|Original Assignee||Eastman Kodak Company|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (14), Referenced by (17), Classifications (15), Legal Events (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present invention relates to solid-state display devices and means to store and display pixel values and images.
Solid-state image display devices utilizing light-emissive pixels are well known and widely used. For example, OLED devices are used in flat-panel displays, in both passive- and active-matrix configurations, and in both top-emitter and bottom-emitter designs. Control circuits for OLED displays are also well known in the art and include both voltage- and current-controlled schemes.
Conventional passive-matrix OLED displays employ a variable current in combination with a fixed period during which the OLED light-emitting element emits light. Successive rows or columns of OLED elements are energized and the entire OLED display is refreshed at a rate sufficient to avoid the appearance of flicker. For example, WO2003034389 A2 entitled “System and Method for Providing Pulse Amplitude Modulation for OLED Display Drivers” published Apr. 24, 2003 describes a pulse width modulation driver for an organic light emitting diode display. One embodiment of a video display comprises a voltage driver for providing a selected voltage to drive an organic light emitting diode in a video display. The voltage driver may receive voltage information from a correction table that accounts for aging, column resistance, row resistance, and other diode characteristics.
Active-matrix OLED devices suffer from manufacturing variability that leads to non-uniformity in OLED displays. Moreover, the OLED light-emitting elements themselves degrade over time and with use, thereby modifying the light output from the devices in response to control and power signals. Hence, conventional active-matrix drive methods that employ stored charge deposited on a local capacitor at each pixel site to control a drive circuit for driving an OLED light-emitting element, will experience an undesirable variation in light output from element to element.
There are a variety of known schemes for compensating for sources of non-uniformity in an active-matrix OLED display, in particular for variations in drive transistor threshold variation. For example, US20040207614 entitled “Display device of active matrix drive type” describes such a pixel-control circuit. U.S. Pat. No. 6,777,888 entitled “Drive circuit to be used in active matrix type light-emitting element array” describes another such design. However, such circuit designs are typically complex and employ many more elements and control signals. Hence, such an approach may reduce manufacturing yields and reduce the light-emissive area of the OLED device.
Because one source of non-uniformity in an OLED display results from variability in the threshold switching characteristics of thin-film drive transistors employed in active-matrix designs, one approach to improving uniformity in an active-matrix OLED display is to employ pulse-width modulation techniques in contrast to charge-deposition control techniques. These pulse-width modulation techniques operate by driving the OLED at a maximum current and brightness for a specific first amount of time and then turning the OLED off for a second amount of time. If the sum of the first and second amounts of time is sufficiently small, the flicker resulting from turning the OLED on and off periodically will not be perceptible to a viewer. The brightness of the OLED element is then controlled by varying the ratio of amount of time that the OLED is turned on in comparison to the amount of time that the OLED is turned off.
A variety of methods for controlling an OLED display using pulse-width modulation are known. For example, U.S. Pat. No. 6,809,710 entitled “Gray scale pixel driver for electronic display and method of operation therefore” granted Oct. 26, 2004 discloses a circuit for driving an OLED in a graphics display. The circuit employs a current source connected to a terminal of the OLED operating in a switched mode. The current source is responsive to a combination of a selectively set cyclical voltage signal and a cyclical variable amplitude voltage signal. The current source, when switched on, is designed and optimized to supply the OLED with the amount of current necessary for the OLED to achieve maximum luminance. When switched off, the current source blocks the supply of current to the OLED, providing a uniform black level for an OLED display. The apparent luminance of the OLED is controlled by modulating the pulse width of the current supplied to the OLED, thus varying the length of time during which current is supplied to the OLED.
By using a switched mode of operation at the current source, the circuit is able to employ a larger range of voltages to control the luminance values in a current-driven OLED display. However, use of current-driven circuits is complex and requires a large amount of space for each pixel in a display device.
There are also methods known for providing both a pulse width control and a variable charge deposition control in a single circuit. U.S. Pat. No. 6,670,773 entitled “Drive circuit for active matrix light emitting device” suggests a transistor in parallel with an OLED element. The described technique, however, diverts driving current from an OLED, thereby decreasing the operating efficiency of the circuit. Other designs employ circuit elements in series with the OLED element for controlling or measuring the performance of the OLED element. For example, WO2004036536 entitled “Active Matrix Organic Electroluminescent Display Device” published Apr. 29, 2004 illustrates a circuit having additional elements in series with an OLED element. However, when placed in series with an OLED element, transistors will increase the overall voltage necessary to drive the OLED element or may otherwise increase the overall power used by the OLED element or decrease the range of currents available to the OLED element.
An additional problem faced by OLED devices is the change in OLED material characteristics as the OLED elements are used. Typically, the OLED elements become less efficient and have a higher effective resistance. Both of these factors tend to increase the voltage needed to drive current through the OLED element. This increases the overall voltage of the system, inhibiting the brightness of the elements at a given voltage.
Thin-film transistors used to drive a typical active-matrix OLED element also place restrictions on operation. A typical transistor has an operating range defined by its current/voltage characteristics. At low voltages or currents, a transistor will no longer operate in a region with a linear response to changes in control signals. Transistor circuits are designed to operate within a restricted range where the performance of the transistor will behave as desired. If control signals move the transistors out of the restricted operating range, the device will no longer behave as desired.
New OLED materials and structures are under development that greatly reduce the current needed to produce a suitable light output. See, for example, U.S. Patent Publication No. 2003/0170491 by Liang-Sheng L. Liao et al., entitled “Providing an Organic Electroluminescent Device Having Stacked Electroluminescent Units”. These structures require changes in driving circuits to provide suitable control within a desired operating region of a transistor circuit.
There is a need therefore for an improved control circuit for active-matrix OLED devices having simplified design, flexible control, and desired operation that does not increase the power used by the circuit.
In accordance with one embodiment, the invention is directed towards an OLED device and control circuit, comprising:
a) an OLED responsive to a drive signal;
b) a drive circuit connected to the OLED responsive to a charge signal for controlling the drive signal;
c) one or more variable charge storage capacitors providing the charge signal;
d) a deposition circuit for depositing variable charge in the one or more variable charge storage capacitors; and
e) a modulation circuit responsive to an external modulation signal for removing charge from the one or more variable charge storage capacitors.
The present invention provides an OLED control device having a simplified control structure while providing improved performance.
In operation, the select 22 and data 24 signals turn on deposition transistor 20 to deposit a charge corresponding to the data deposition signal 24 in capacitor 16. The external modulation signal 17 is held low for this operation so that the charge modulation transistor 18 is off. When the capacitor 16 is charged, the drive transistor 14 is proportionally turned on to provide a current flow from the power signal Vdd, through the drive transistor 14 and the OLED 12 to the cathode voltage CV, thereby causing the OLED to emit an amount of light corresponding to the charge on the capacitor 16. After a period of time, the external modulation signal 17 is raised to a high voltage, thereby turning on the charge modulation transistor 18 and causing the charge in the capacitor 16 to drain out. With no, or reduced, charge in the capacitor 16, the drive transistor 14 will provide less current to the OLED 12, thereby reducing the light output of the OLED. In one embodiment, the external modulation signal 17 may be used to completely drain the charge in the capacitor 16 so that the drive transistor 14 is turned off and no current flows through the OLED 12, effectively turning it off. In an alternative embodiment, the external modulation signal 17 may be used to partially drain the charge in the capacitor 16 so that the drive transistor 14 is controlled at one or more selected levels to modify the current flow through the OLED 12, effectively modifying the light output in response to the modulation signal 17.
In a conventional, prior-art flat-panel display, a display signal is typically refreshed periodically at a rate high enough to provide the appearance of smooth motion in sequential frames of a video stream. Refresh rates are typically 30, 60, 70, 75, 80, 90, or 100 frames per second for monitors, 50 or 60 frames per second for televisions, and 24 frames per second for films. Hence, in a conventional flat-panel display, the charge in the variable charge storage circuit 15 is updated at the selected refresh rate appropriate to the application. According to the present invention, after the charge has been refreshed for an OLED element within a given frame, the external modulation signal 17 is applied to reduce the charge in the variable charge storage circuit 15 and change the light output of the OLED 12 before the next time the charge is refreshed in response to the following frame update.
In one embodiment of the present invention, when a desired average brightness over a frame period is desired, the charge deposited in the variable charge storage circuit 15 to drive the drive circuit 30 and OLED 12 is increased above the level necessary to provide an average brightness over the entire refresh period during a first portion of the frame refresh period. During the second portion of the frame refresh period, the charge deposited in the variable charge storage circuit 15 is decreased in response to the external modulation signal 17. Hence, the OLED has a variable brightness during the frame refresh period. Preferably, the refresh period is sufficiently short that the variation in brightness through the period is not perceptible to a viewer. The average brightness of the OLED device is perceived to be the total amount of light emitted during the refresh period. If P1 is defined as the first portion of the frame period P during which the OLED emits light at a brightness V1, and P2 is defined as the second portion of the frame period during with the OLED emits light at a brightness V2, and P1+P2 equals P, then the average brightness can be calculated as ((V1*P1)+(V2*P2))/P.
According to various embodiments of the present invention, the use of an external modulation signal 17 in combination with the control signals 22 and 24 as described provides various benefits. First, this design does not require the use of a control transistor in series with the OLED element itself (which series element would increases the voltage (Vdd) necessary to drive the OLED, thereby decreasing the efficiency of the system), or a current-diverting transistor in parallel with the OLED (which parallel element diverts current, thereby decreasing the efficiency of the system), while still providing a means to drive an active-matrix OLED element at a variety of levels within a single period.
Second, referring to
While the aging of OLED elements can reduce the current through an OLED element and cause driving transistors to problematically operate in a non-linear region, new materials and OLED structures may also have a similar effect. Applicant has demonstrated materials that are much more efficient at producing light in response to current than those commercially available today. Moreover, stacked structures can greatly reduce the current needed to produce a suitable light output. See, for example, U.S. Patent Publication No. 2003/0170491 referenced above. Hence, these materials and structures require much lower currents than are used in commercial applications today.
The use of a combination of variable brightness and variable duration provides a simple technique for operating low-current OLED materials and structures. Instead of operating in non-preferred operating regions to get a desired light output for such low-current devices, by providing a higher current to the OLED during a first portion of a refresh cycle, driving transistors may be operated in their preferred operating region during such first portion. During the remaining second portion of the period, the OLED may not be driven at all, and hence need not be driven in a non-preferred operating region.
In a typical pulse-width modulation scheme of the prior art, an OLED is driven at a constant, high brightness for a data-dependant variable portion of a period. In this scheme, data must be written at least twice in every period, to turn the OLED on and then off again. This scheme also requires that a large OLED drive current be used, reducing the lifetime of the materials, and that a complex, very high rate control signal be employed to control the variable pulse width. The variable pulse width must be controlled to within at least one 256th of a period to support an 8-bit gray scale display. This can be difficult to accomplish. By combining a variable charge and a variable period, the OLED drive current necessary may be reduced and the pulse width control may be greatly simplified. For example, data may be written only once, and only a few different pulse widths may be used, such as two or four. Hence, a third advantage of the present invention is simplified control. The variable charge deposited in the capacitor(s) may be modified to accommodate changes in the pulse widths.
The present invention may also be employed to compensate for changes in the operating characteristics of an OLED element. As OLED are used, their efficiency drops and resistance increases. By extending the length of the first portion of the refresh period with respect to the second portion of the refresh period, more light may be emitted by the device, thereby compensating for the reduced light output efficiency of the OLED element. In this case, the change in efficiency of the OLED materials directly affects the change in P1. For example, the ratio R of P1 to P may be increased as the relative efficiency ET at time T of the OLED materials decreases. Over time, as the efficiency of the OLED materials decreases, ET Will likewise decrease, and R may be increased to R=Rinitial/ET, where Rinitial is the initial ratio of P1 to P. R cannot exceed 1.0 (the point of maximum compensation possible), and ET is <=1.0.
The initial brightness value V1 of the control signal during the first portion P1 may be a predetermined maximum value VMax corresponding to the desired brightness of the OLED elements divided by R and representing a maximum quantity of light output from the light-emitting elements for P1. VMax and P1 will be set based on the desired brightness, the desired lifetime of the OLED display, and the lifetime of the light-emitting materials in the OLED.
As is known, deposit-and-hold circuits such as may be found in active matrix OLED display devices may lead to perceptual blurring if an observer's eye attempts to track a moving object across the display device screen. By modulating the charge in the variable storage capacitor 16 to reduce the length of time the OLED is emitting light, this blurring effect may be reduced. The present invention thus may be employed to more simply reduce motion artifacts in such display devices.
The present invention may be employed in a display having a plurality of OLED light-emitting elements and associated active-matrix circuits. These light-emitting elements may be organized in rows and columns and the control signals supplied to them may drive rows or columns at a time. The external modulation signal may be connected to all of the OLED elements in common, so that a single control structure operates all of the modulation circuitry. Alternatively, separate modulation signals may be employed for groups of OLEDS, for example groups may comprise all of the OLED elements that emit light of a particular color in a color display. Since different OLED materials are employed in a color display to emit different colors and age at different rates, it can be advantageous to control each OLED color-element grouping separately. Typically, the data and select control signals refresh lines or columns in a display at a time; the same method of cycling through the rows or columns may be employed to control the modulation signal so that each OLED commonly connected to a modulation signal will be updated one row or column at a time and cause the OLED to emit light for the same amount of time.
An OLED controller suitable for use with the present invention can be constructed using conventional digital logic control methods. The circuit control signals may be applied using conventional designs.
In a preferred embodiment, the invention is employed in an emissive display that includes Organic Light Emitting Diodes (OLEDs) which are composed of small molecule or polymeric OLEDs as disclosed in but not limited to U.S. Pat. No. 4,769,292, issued Sep. 6, 1988 to Tang et al., entitled “Electroluminescent Device with Modified Thin Film Luminescent Zone” and U.S. Pat. No. 5,061,569, issued Oct. 29, 1991 to VanSlyke et al., entitled “Electroluminescent Device with Organic Electroluminescent Medium”. Many combinations and variations of OLED materials and architectures are available to those knowledgeable in the art, and can be used to fabricate an OLED display device according to the present invention.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US6670773||Sep 20, 2002||Dec 30, 2003||Canon Kabushiki Kaisha||Drive circuit for active matrix light emitting device|
|US6738031 *||May 22, 2001||May 18, 2004||Koninklijke Philips Electronics N.V.||Matrix array display devices with light sensing elements and associated storage capacitors|
|US6753655 *||Dec 30, 2002||Jun 22, 2004||Industrial Technology Research Institute||Pixel structure for an active matrix OLED|
|US6777888||Sep 20, 2002||Aug 17, 2004||Canon Kabushiki Kaisha||Drive circuit to be used in active matrix type light-emitting element array|
|US6809710||Jan 22, 2001||Oct 26, 2004||Emagin Corporation||Gray scale pixel driver for electronic display and method of operation therefor|
|US20010055008 *||May 22, 2001||Dec 27, 2001||U.S. Philips Corporation||Matrix array display devices with light sensing elements and associated storage capacitors|
|US20030170491||Feb 15, 2002||Sep 11, 2003||Eastman Kodak Company||Providing an organic electroluminescent device having stacked electroluminescent units|
|US20040056604 *||Dec 30, 2002||Mar 25, 2004||Jun-Ren Shih||Pixel structure for an active matrix OLED|
|US20040207614 *||Jan 20, 2004||Oct 21, 2004||Atsuhiro Yamashita||Display device of active matrix drive type|
|US20040227706 *||Mar 1, 2004||Nov 18, 2004||Chao-Chin Sung||Driving method for active matrix oled display|
|US20050105031 *||Mar 29, 2004||May 19, 2005||Po-Sheng Shih||[pixel structure of display and driving method thereof]|
|US20060028407 *||Aug 5, 2005||Feb 9, 2006||Chen-Jean Chou||Light emitting device display circuit and drive method thereof|
|WO2003034389A2||Oct 17, 2002||Apr 24, 2003||Clare Micronix Integrated Syst||System and method for providing pulse amplitude modulation for oled display drivers|
|WO2004036536A1||Oct 8, 2003||Apr 29, 2004||Koninkl Philips Electronics Nv||Active matrix organic electroluminescent display device|
|Citing Patent||Filing date||Publication date||Applicant||Title|
|US8115707 *||Jun 28, 2005||Feb 14, 2012||Ignis Innovation Inc.||Voltage-programming scheme for current-driven AMOLED displays|
|US8120555||Nov 2, 2007||Feb 21, 2012||Global Oled Technology Llc||LED display with control circuit|
|US8664644||Apr 19, 2011||Mar 4, 2014||Ignis Innovation Inc.||Pixel driver circuit and pixel circuit having the pixel driver circuit|
|US8816946||Feb 7, 2014||Aug 26, 2014||Ignis Innovation Inc.||Method and system for programming, calibrating and driving a light emitting device display|
|US8890220||Sep 26, 2013||Nov 18, 2014||Ignis Innovation, Inc.||Pixel driver circuit and pixel circuit having control circuit coupled to supply voltage|
|US8963814 *||Jun 23, 2009||Feb 24, 2015||Samsung Display Co., Ltd.||Organic light emitting display device and method of driving the same|
|US8994625||Jan 16, 2014||Mar 31, 2015||Ignis Innovation Inc.||Method and system for programming, calibrating and driving a light emitting device display|
|US9059117||Jul 3, 2014||Jun 16, 2015||Ignis Innovation Inc.||High resolution pixel architecture|
|US9070775||Apr 4, 2014||Jun 30, 2015||Ignis Innovations Inc.||Thin film transistor|
|US9093028||Dec 2, 2010||Jul 28, 2015||Ignis Innovation Inc.||System and methods for power conservation for AMOLED pixel drivers|
|US9093029||Jul 25, 2013||Jul 28, 2015||Ignis Innovation Inc.||System and methods for extraction of threshold and mobility parameters in AMOLED displays|
|US9111485||Mar 16, 2013||Aug 18, 2015||Ignis Innovation Inc.||Compensation technique for color shift in displays|
|US20100020051 *||Jan 28, 2010||Do-Ik Kim||Organic light emitting display device and method of driving the same|
|US20100191492 *||May 5, 2009||Jul 29, 2010||Samsung Mobile Display Co., Ltd.||Flicker detecting device and flicker detecting method using the same, and recording medium storing computer program for executing the flicker detecting method|
|USRE45291 *||Nov 26, 2013||Dec 16, 2014||Ignis Innovation Inc.||Voltage-programming scheme for current-driven AMOLED displays|
|CN101884061B||Nov 3, 2008||Sep 11, 2013||全球Oled科技有限责任公司||LED display with control circuit|
|WO2009058393A2 *||Nov 3, 2008||May 7, 2009||Eastman Kodak Co||Led display with control circuit|
|U.S. Classification||315/169.1, 345/76|
|International Classification||G09G3/30, G09G3/10|
|Cooperative Classification||G09G2300/0842, G09G2300/0876, G09G2310/0251, G09G2320/043, G09G3/2011, G09G2360/147, G09G2320/0261, G09G3/3233, G09G2320/0271, G09G3/2014|
|Apr 8, 2005||AS||Assignment|
Owner name: EASTMAN KODAK COMPANY, NEW YORK
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:COK, RONALD S.;REEL/FRAME:016461/0209
Effective date: 20050407
|Jan 22, 2010||FPAY||Fee payment|
Year of fee payment: 4
|Feb 26, 2010||AS||Assignment|
Owner name: GLOBAL OLED TECHNOLOGY LLC,DELAWARE
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:EASTMAN KODAK COMPANY;REEL/FRAME:023998/0368
Effective date: 20100122
Owner name: GLOBAL OLED TECHNOLOGY LLC, DELAWARE
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:EASTMAN KODAK COMPANY;REEL/FRAME:023998/0368
Effective date: 20100122
|Jan 8, 2014||FPAY||Fee payment|
Year of fee payment: 8